1. Field of the Invention
This invention relates to a method of producing Teflon electrets having an internal positive charge and improved charge stability, and devices made therefrom.
2. Description of the Prior Art
Electrets are dielectric materials having a net electric charge in at least a portion of the material or on a surface thereof. Although electrets are sometimes defined as having an electric polarization (i.e., an equal number of positive and negative charges separated by a distance), the term as used in the art also includes dielectric materials having a net positive or negative charge. When made of polymer material such as Teflon, electrets are frequently used as electromechanical transducers for microphones, earphones, speakers, etc. Teflon, a registered trademark of E. I. Dupont Company for solid, polymerized CF.sub.2, is a common material for making an electret, due to its ability to retain a high degree of surface charge over a period of time. One typical method of charging a Teflon electret material is by means of a corona discharge technique, wherein an ionized gas forms one electrode to the Teflon being charged; see, for example, U.S. Pat. No. 3,702,493. By this and other methods, including penetrating ion or electron beam charge injection, an internal excess charge of the material can be achieved. In addition, a surface charge can be applied to the electret by conventional techniques. The charges on opposite surfaces may be of the same or opposite polarity.
It is further recognized in the prior art that when charging a Teflon electret, as by the corona ionization process, the stability of the charge on the electret can be improved by charging at an elevated temperature; see, for example, "Polymer Electrets Corona-Charged At High Temperature", by S. S. Bamji et al, in the Journal of Electrostatics, Vol. 6, pages 373-379, (1979). However, for producing a stable internal charge in Teflon, the prior art knew only the introduction of negative charge into the material. It has been widely believed that positive internal charges are inherently unstable in Teflon material; see, for example, "Thermally Stimulated Discharge Of Polymer Electrets", by J. V. Turnhout, Elsevier Publishers, Amsterdam, Oxford, and New York, (1975), at pages 267-268. As noted in that reference, it was conventionally believed that the observed much higher mobility of positive charge carriers in Teflon made positively internally charged Teflon inherently less stable than negatively charged Teflon electrets. This was due in part to the belief that electrons could be more stably trapped in deep energy traps in Teflon, because fluorine atoms are highly electronegative.
However, it is highly desirable to also obtain Teflon electrets having a stable positive internal charge. This would allow new applications in electromechanical transducers, electrostatic motors, electrostatic air filters, etc. In particular, the design of a push-pull type electrostatic headphone is simplified if an electrostatic element can contain both positive and negative charges. To overcome the poor internal charge stability of positively charged electrets, recourse has been made to try to obtain stable surface charges. For example, it has been found that positive surface charges can be more stably produced on Teflon electrets if the surface is corrugated; see, for example, "Plate Electrets And Their Use In Condenser Type Headphones", by S. Tamura et al in Charge Storage, Charge Transport, And Electrostatics With Their Applications, edited by Y. Wada et al, at pages 128-132, (1979). The surface-charging technique avoids the problem of high mobility of positive charges in the internal portion of the material, since the surface trapping states are different than the internal trapping states.
However, surface charges are typically limited to a depth of about 4 microns in Teflon material. In order to obtain a strengthened electrostatic charge, and allow for new device configurations, it would be desirable to be able to obtain a stable internal positive charge in Teflon material.